btrfs: wait between incomplete batch memory allocations

When allocating memory in a loop, each iteration should call
memalloc_retry_wait() in order to prevent starving memory-freeing
processes (and to mark where allocation loops are). Other filesystems do
that as well.

The bulk page allocation is the only place in btrfs with an allocation
retry loop, so add an appropriate call to it.
Signed-off-by: NSweet Tea Dorminy <sweettea-kernel@dorminy.me>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -6,6 +6,7 @@
 #include <linux/mm.h>
 #include <linux/pagemap.h>
 #include <linux/page-flags.h>
+#include <linux/sched/mm.h>
 #include <linux/spinlock.h>
 #include <linux/blkdev.h>
 #include <linux/swap.h>
@@ -3153,6 +3154,9 @@ int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array)
 
 		allocated = alloc_pages_bulk_array(GFP_NOFS, nr_pages, page_array);
 
+		if (allocated == nr_pages)
+			return 0;
+
 		/*
 		 * During this iteration, no page could be allocated, even
 		 * though alloc_pages_bulk_array() falls back to alloc_page()
@@ -3160,6 +3164,8 @@ int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array)
 		 */
 		if (allocated == last)
 			return -ENOMEM;
+
+		memalloc_retry_wait(GFP_NOFS);
 	}
 	return 0;
 }